No hot water in the shower

Is it only the shower, or is the whole flat cold?

Everything below splits along this line, so do the test properly. Run the hot tap at the nearest basin for a full minute. A long pipe run from an airing cupboard to a bathroom holds a surprising amount of cold water, and people write off a good cylinder after ten seconds of a tap that had no time to clear.

If the tap runs hot and the shower does not, the hot water is being made and delivered correctly and something between the hot supply and the shower head is stopping it. That is a shower valve fault, and it is the common case by a wide margin. The rest of the house behaves normally, which is exactly why it is confusing.

If the tap runs cold as well, stop looking at the shower. You have no hot water anywhere and the cause is the appliance making it: a boiler that is not firing for hot water, a cylinder whose immersion or coil has stopped heating, a failed cylinder thermostat, or a timer that has quietly lost its programme after a power cut. That is a different fault with a different route through it, and if the radiators are still hot while every tap is cold, the page on no hot water but the heating works takes that case apart.

There is a third case. The tap runs hot briefly, the shower goes warm then cold, and a bath will not fill hot either. That is a cylinder holding only a partial charge, and the shower notices first because it is the biggest draw in the property. Where the shower is warm rather than cold and every tap is doing the same thing, the page on water not getting hot is the better starting point.

• Hot tap hot, shower cold: shower valve, cartridge or check valves.
• Hot tap cold, shower cold: boiler, cylinder, immersion or controls.
• Everything runs hot briefly then cools: the cylinder is only partly heated.
• Electric shower cold while every tap in the flat is hot: the shower is an appliance in its own right and the rest of the plumbing tells you nothing about it.

Hot at the taps, cold in the shower: the mixer valve

A mixer shower is a small blending valve with two inlets, hot on one side and cold on the other. Inside it there is a cartridge that decides the ratio, and behind each inlet there is a check valve, a small spring loaded non-return valve whose only job is to stop water from one side pushing back into the other. Those check valves are the single most common reason a shower runs cold while the taps are hot, and almost nobody thinks of them.

The mechanism is worth understanding because it explains the symptom exactly. Cold mains arrives at the valve at a much higher pressure than the hot side, particularly where the hot comes from a cylinder rather than a combi. The check valve on the cold inlet holds it back. When that check valve scales up, perishes or sticks open, cold water crosses into the hot inlet and travels back up the hot pipe. What comes out of the shower head is cold or barely warm no matter where you set the handle, because the hot supply at the valve is no longer hot: it is full of cold that has pushed in behind it.

You can usually confirm this without tools. Turn the shower to full hot, run it for a minute, then go and open the hot tap at the basin. If that tap, which was hot five minutes ago, now runs cool for a while before recovering, cold is crossing into the hot pipework at the shower. Some people also notice the cold tap in the bathroom running slightly warm, which is the same fault in the other direction.

The repair is the two check valves, which on a bar mixer live in the inlet elbows behind the wall plate and on a concealed valve sit in removable cartridges on the inlet spigots. They are cheap parts. The work is in getting at them and resealing the valve afterwards, so an otherwise sound shower is worth repairing rather than replacing.

Before anyone opens a shower valve, find the isolating valves for it. In most flats they are on the hot and cold pipes feeding the shower, often behind an access panel, under the bath, or in the airing cupboard. If there are none, the whole supply has to come off, and on an unvented system that means shutting the mains and draining down rather than turning a screwdriver.

Why a thermostatic shower cartridge fails towards cold

A thermostatic shower valve holds a wax element that expands and contracts with temperature and drives a shuttle between the hot and cold ports. It is a mechanical thermostat with no electronics in it, and it is designed to fail safe: if the cold supply is lost the element shuts the hot side down rather than let you be scalded. That safety behaviour is why so many failed thermostatic showers run cold and not hot. The valve has decided something is wrong and closed the hot port.

Three things go wrong with them. The wax element loses its travel and stops responding, so the shower sits at one temperature regardless of the handle. The shuttle and its seals seize in the body, usually with scale, and jam towards the cold end. Or the small filters in the inlets block with debris, the hot flow drops away, and the valve compensates by cutting the hot back further still.

The symptom tells you which of the three you have. Stuck lukewarm at any handle position is a dead element. Fine yesterday and cold today, often with a new noise, is a seized shuttle or a check valve. Gradually weaker and cooler over months is scale, in the cartridge or the inlet filters, and the falling flow rate will have been warning you for a while.

Most thermostatic cartridges are a replaceable part rather than a reason to change the valve, provided the brand still supports the model. The cartridge is a moulded assembly that pulls out once the handle, the shroud and the retaining clip or nut are off. The honest caveat is that on an older valve the cartridge can be seized into the body hard enough that removing it damages the body, and on some discontinued ranges the cartridge is no longer made. In those cases the valve is replaced, and if it is a concealed valve that means tiling work.

One thing to rule out before condemning a cartridge: many thermostatic valves have a maximum temperature stop, a ring or a button on the handle that limits how far towards hot it will turn. If someone has had the handle off, or the shower has been serviced or descaled, that stop can be reassembled in the wrong position. The shower then works perfectly but will not go above tepid. Pull the handle cap off, look for the stop ring, and check it before spending money.

• Stuck lukewarm at every handle position: thermostatic element has failed.
• Sudden change to cold, sometimes with a new noise: seized shuttle or failed check valve.
• Gradual loss of heat and flow together over months: scale in the cartridge or inlet filters.
• Works but will not reach a decent temperature: the maximum temperature stop may simply be set wrong.

Electric shower running cold on every setting

An electric shower is not plumbed into your hot water at all. It takes cold mains, runs it through a small heater tank with one or two elements in it, and heats it as it passes. Nothing about the rest of your plumbing applies. A flat with perfect hot water at every tap can have a stone cold electric shower, and the cause is inside the unit or on its electrical supply.

Start with the supply, because it is free to check. Electric showers run on their own circuit with a dedicated breaker or an RCBO in the consumer unit, and most have a ceiling mounted pull cord isolator outside or just inside the bathroom door. If the shower runs cold water at full flow but the neon on the front is dead and no setting makes any difference, the unit is getting no power. Check the pull cord is actually in the on position, then check the breaker. A breaker that trips again as soon as it is reset means a fault inside the shower, commonly a failed element leaking to earth, and that is where you stop and call someone.

If the unit has power and still runs cold, the usual culprits are the heating element, the thermal cut out, and the flow or pressure switch. A failed element is the classic: the shower behaves normally, water flows at the right rate, the settings click over, and nothing gets warm. On a dual element unit you get a more revealing symptom, which is that the low setting works and the high does not, or the other way round, because only one of the two elements has gone.

The thermal cut out is a safety device that opens when the heater tank overheats, and it is worth knowing about because it usually means something else is wrong. Tanks overheat when flow through them collapses, and flow collapses because the inlet filter is blocked, the shower head is scaled, or the hose is furred up. Reset the cut out without fixing the flow and it trips again within days. On some models the cut out is a one shot part that must be replaced rather than reset.

The flow switch, often a pressure operated microswitch, tells the shower that water is moving and the elements can be energised. If mains pressure has dropped or a stop tap has been left half shut, the switch never closes and you get cold water at what still looks like acceptable flow.

Nothing inside an electric shower should be opened by anyone who is not qualified for it. The unit contains mains voltage in immediate contact with water, the element terminals stay live when the pull cord is in the wrong position on incorrectly wired installations, and a shower that trips its breaker is telling you there is an earth fault. Pull the cord, switch the breaker off and leave the cover on.

The shower runs hot then goes cold after a few minutes

This is a different fault from a shower that never gets hot, and it points upstream rather than at the valve. Something is providing hot water for a short period and then running out or shutting down, and the shower is the biggest continuous draw in the property so it is where you notice.

On a stored hot water system, the plain answer is that the cylinder has less hot water in it than you think. A cylinder heated only by the top immersion holds a fraction of its capacity, because an immersion only heats the water above it. If someone has been running on the boost switch, or the lower immersion has failed, or the boiler only ran a short hot water window that morning, you get four or five minutes of genuinely hot water followed by cold. The giveaway is that a bath will not fill hot either.

On a combi boiler, a shower that goes cold after a couple of minutes usually means the boiler is failing to sustain hot water production rather than failing to start it. A scaled plate heat exchanger gives exactly this: plenty of heat for the first minute while the plates are still holding warmth, then a steady fade as the scaled passages cannot transfer heat fast enough at flow rate. A diverter valve that is sticking part way gives a cruder version of the same thing, where the hot water dies the moment the heating calls. The appliance side of that is set out in full on our page for no hot water from a combi boiler.

There is also a purely hydraulic version of this. If the hot side loses pressure or flow while the cold side does not, a thermostatic valve holds temperature for a while by cutting the cold back, then runs out of room and goes cold while every other outlet is fine. A failing shower pump or a half closed isolating valve on the hot inlet shows up here long before it shows up anywhere else.

Why limescale makes the same shower go cold again

Most of London sits on hard water, and a shower is the worst place in the house for it because scale forms fastest where hard water is heated and moved at the same time. Every part of a shower that is affected by scale produces a version of the no hot water complaint.

In a thermostatic cartridge, scale builds on the shuttle and in the bore it slides in. The valve loses its range first, so the usable band of the handle gets narrower, and then it seizes, usually towards cold. In the inlet filters, scale and the small fragments it sheds pack the mesh until the hot flow through the valve drops below what the thermostat needs to work with. In an electric shower, scale coats the element itself, which insulates it from the water it is supposed to be heating, drives the tank temperature up and eventually trips the thermal cut out.

The head and hose are the easiest to deal with and the most often ignored. A furred head raises back pressure and cuts flow, which upsets a thermostatic valve and overheats an electric one. Soak the head in a descaler and clear the holes. A hose that rattles when you shake it is full of loose scale and is cheaper to replace than to fight.

Where it keeps happening, a scale reducer on the supply is worth fitting. For most people the practical answer is servicing: descale the head, clean the inlet filters, and treat a thermostatic cartridge as a consumable part on a hard water main. What scale actually does further upstream, inside a cylinder and a heat exchanger, is covered in our piece on hard water and hot water cylinders in London.

When it really is the cylinder or the boiler, not the shower

If the hot taps are cold too, the shower is a symptom rather than the fault. On a stored system the questions are whether the cylinder is being heated at all and by what. An unvented or vented cylinder normally has two heat sources, a coil fed by the boiler and one or two electric immersion heaters, and it is useful to know which one you have been relying on.

A cylinder that is cold to the touch all the way up, with the boiler running happily for the radiators, points at the cylinder thermostat, the two port motorised valve that lets boiler flow into the coil, or the wiring between them. A cylinder that is hot at the top and cold at the bottom is being heated by the top immersion only. A cylinder that is warm but never hot, on a system that used to be fine, is often a coil that has scaled or a boiler running at too low a flow temperature for the cylinder to reach its stat. The hot water tank not heating page follows that chain from the stat to the coil, and the immersion heater not working page covers the electric side of the same cylinder.

On a combi there is no storage at all, so a total loss of hot water with working heating narrows quickly to the diverter valve, the flow sensor that tells the boiler you have opened a tap, or the plate heat exchanger. The clean way to tell them apart is what happens when you open a hot tap: if the boiler does not fire at all, it is not seeing the demand, which is the flow sensor or the diverter microswitch. If it fires and the water still comes out cold, it is making heat and failing to move it into the domestic side.

Neither of those is a shower problem and neither is worth chasing at the shower valve. It is worth saying plainly because people replace a perfectly good shower and then discover the hot tap was cold as well.

What to check yourself before calling anyone

A fair proportion of the shower calls we take are things the owner could have found in ten minutes with a torch and a screwdriver. Work through these before you book anything, and if the taps turn out to be cold as well, switch to our no hot water checklist before you call.

If all of that checks out and the shower is still cold while the taps are hot, the fault is inside the valve or the unit and that is the point where the visit earns its money.

• Run the nearest hot tap for a full minute. This is the test that decides everything else.
• Unscrew the shower head and run the shower without it. If the temperature is suddenly fine, the head is scaled and that is the whole fault.
• Check the maximum temperature stop under the handle cap on a thermostatic valve, particularly if anyone has had the handle off.
• Find the isolating valves on the shower supplies and confirm both are fully open. A hot side isolator knocked half shut during other work is a common cause.
• On an electric shower, confirm the ceiling pull cord is on and the breaker in the consumer unit has not tripped.
• On a stored system, check whether the immersion switch has been left off and whether the timer or programmer is still holding the right time after a power cut.
• Open the cold tap in the bathroom and feel whether it runs slightly warm, which suggests a failed check valve in the shower mixer.

If the shower trips the breaker or the RCD when you switch it on, stop there, leave it off and get it looked at. That is an earth fault in an appliance that puts mains voltage next to running water, and it is not something to keep resetting.

What an engineer does on the visit

The order on site follows the same logic as the test at the top of this page, because it clears whole categories of cause rather than guessing at parts. The first thing done is a temperature check at the nearest outlets with a thermometer rather than a hand, to establish whether hot water is arriving at the room at all and at what temperature. That single reading separates a supply fault from a shower fault before anything is dismantled.

If the supply is good, the shower valve comes apart. The inlet filters are pulled and inspected, the check valves are tested, and the cartridge is removed and looked at for scale and for travel in the element. Most faults are visible at that point. If the valve is a brand and model still in parts support, the cartridge and check valves are changed and the valve is reassembled, flushed and tested across its range with a thermometer.

On an electric shower the sequence is electrical. The supply is proved at the isolator and at the unit, then the element continuity and insulation resistance are measured, the thermal cut out is tested, and the flow or pressure switch is checked for operation at the available flow rate. An element that reads open circuit or fails insulation resistance is condemned on the spot, and on most modern units replacing the whole shower is more sensible than rebuilding it.

If the fault is upstream, the work moves to the cylinder stat and motorised valve on a stored system, or the diverter valve, flow sensor and plate heat exchanger on a combi. Better found early than after a tiled valve is out of the wall.

Our engineers are qualified for the work described here, including the unvented hot water side where a cylinder turns out to be the cause. The rate is fixed before we attend and the total depends on how long the job takes, so being able to tell us whether the taps are hot when you call genuinely shortens the visit.